1. Field of the Invention
The present invention relates to a circuit for shifting a level between two bi-directional signals having different voltage levels that are usable for such optical transceivers in optical communication networks. More particularly, the present invention relates to an I2C interface technique using such a circuit.
2. Description of the Related Art
In recent days, there has been a sudden increase of Internet users, as well as a rapid expansion of optical communication networks for third generation mobile communication. This increase in usage has created a demand for a variety of optical communication modules to accommodate the additional users, particularly those using third generation mobile communication equipment. In addition, so long as optical communication markets strive to create/expand long distance toll networks to connect one nation to another, or one city to another, the increased demand has gradually spread to local networks or subscriber networks. In fact, the demand for the optical communication modules is increasing daily.
Optical transceivers employed for such optical communication networks are essential parts of optical communication networks. Optical transceivers include a transmission module for converting electrical signals, such as voices, images and data, into optical signals to be transmitted via optical fibers, and a reception module for restoring received optical signals to the electrical signals. Typically, the transmission module and reception module are integrated in one unit. There are many types such optical transceivers, examples of which include the SFP (2.5G Small Form-factor Pluggable) level transceivers, and a more recent XFP (10G Small Form-factor Pluggable) level transceivers. The XFP level transceiver is widely used in schools, cyber apartments, etc., and the market for such items is growing.
FIG. 1 is a schematic block diagram of a general optical transceiver employing a bi-directional signal level shift circuit for an I2C interface. I2C is a 2-wire serial interface standard that only requires two lines (clock and data) for full duplexed communication between multiple devices.
Referring to FIG. 1, the optical transceiver includes a receiver optical sub assembly (ROSA) 128 for receiving an optical signal and converting the optical signal into an electrical signal, an electrical signal adjusting unit 129 for performing an appropriate adjustment operation for the electrical signal output from the ROSA 128, a laser output unit (EML: Electro-absorption Modulated Laser) 118 for generating an optical signal to be transmitted based on a driving signal, an optical signal adjusting unit 119 for performing an appropriate adjustment operation for the optical signal to be transmitted, and an EML driving unit 116 for outputting the driving signal to the EML 118 based on a signal output from the optical signal adjusting unit 119.
In addition, the optical transceiver of the type shown in FIG. 1 further includes a control information interface 110 for exchanging control information and signals for network control with a central controller (not shown). The control information interface 110 outputs a variety of operational setting signals to the EML driving unit 116. For example, the control information interface provides operational setting signals to the EML driving unit for internal individual function units, based on corresponding received control information. At the present time, the EML driving unit 116 can be directly provided with a corresponding operational setting signal through a digital-to-analog converter (DAC) 114 if a level of power used in the EML driving unit 116 is equal to the level of power that is used in the control information interface 110. However, if the levels of power used in the EML driving unit 116 and the control information interface 110 are different, it is necessary to provide a signal level shift circuit, such as an optical coupler signal shift/transfer unit 112, in order to make the power levels equal. FIG. 1 shows an arrangement of the optical coupler signal shift/transfer unit 112 in connection between the control information interface 110 and the EML driving unit 116, with the digital-to-analog converter 114 arranged between the optical coupler signal shift/transfer unit 112 and the EML driving unit.
With regard to FIG. 1, the I2C (Inter-IC) communication system can be used as a communication system between the control information interface 110 and the central controller. The I2C communication system, which was originally developed by Philips Co. in Holland, is a synchronous bi-directional two-line communication system using two lines of serial data (SDA) and serial clock (SCL). An operating voltage level used in the I2C communication system is an LVTTL (Low Voltage Transistor-Transistor Logic ) level (+3.3V) and, when the EML is used as a light source of the laser output unit 118, a level of operating voltage of the EML driving unit 116 is −5.2V. The control information interface 110 adjusts an operating point modulation voltage, a pulse width and an offset voltage of the EML driving unit 116 by means of the DAC 114. The DAC 114 receives the data according to the I2C communication system from the control information interface 110 and outputs the setting voltages. Since the I2C communication system uses the LVTTL level (+3.3V) and the operating voltages of the EML driving unit 116 and the DAC 114 is −5.2V, the optical transceiver using the system shown in FIG. 1 includes the signal shift circuit, i.e., the optical coupler signal shift/transfer unit 112 for matching signal levels between the control information interface 110 and the DAC 114. Thus, the optical coupler signal shift/transfer unit overcomes inoperability of the optical transceiver due to a level difference between signals of the EML driving unit and the control information interface.
FIGS. 2 and 3 are detailed circuit diagrams of the optical coupler signal shift/transfer unit 112 shown in FIG. 1. In particular, FIG. 2 shows a circuit for bi-directional SDA signal level shift and FIG. 3 shows a circuit for unidirectional SCL signal level shift. An optical coupler 22, 23/33 shown in FIGS. 2 and 3 may be formed of 6N139 chips, which are available from ‘Fairchild Semiconductor’ Co.
Referring to FIG. 2, which illustrates the circuit for providing a bi-directional SDA signal level shift, a first operating voltage VccA (+3.3V) is input to a light emitting device of a first optical coupler 22 (typically terminal 2 of a 6N139 chip) of optical coupler 22 via a resistor R2 and an output terminal (terminal 3 of 6N139 chip) of the light emitting device is connected to a SDA_1 stage and a ground via a resistor R4. A first diode D1 is connected in a reverse-biased direction to the VccA between the input terminal and the output terminal of the light emitting device of optical coupler 1. Also, a second operating voltage VccB (−5.2V) is input to an SDA_2 stage and an output terminal (terminal 3 of 6N139 chip) of optical coupler 2 via a resistor R3 and an input terminal (terminal 2 of 6N139 chip) of optical coupler 2 is connected to a ground via a resistor R1. A second diode D2 is connected in a direction backward to the ground between the input terminal and the output terminal of the light emitting device of optical coupler 2.
On the other hand, a light receiving device of optical coupler 22 is configured to connect/disconnect the input terminal of the light emitting device of optical coupler 23 to/from the operating voltage, −5.2V, by performing an on/off switching based on the presence or absence of light emitted from the light emitting device. In other words, 6N139 terminals 5 and 6 of optical coupler 22 are connected respectively to the operating voltage of −5.2V and the ground via the resistor R1. In addition, a light receiving device of optical coupler 23 is configured to connect/disconnect the input terminal of the light emitting device of optical coupler 22 to/from the operating voltage of +3.3V by performing an on/off switching based on the presence or absence of light emitted from the light emitting device. In other words, 6N139 terminals 5 and 6 of optical coupler 23 are connected respectively to the ground and the operating voltage of +3.3V via the resistor R2.
With regard to the operation of the SDA signal level shift circuit as shown in FIGS. 2 and 3, when a signal is transmitted from the SDA_1 stage to the SDA_2 stage, if a signal SDA_1 has a high logic (+3.3V), light is not emitted from the light emitting device of optical transceiver 1, and accordingly, a signal SDA_2 goes to a logic high (a voltage resulting from the division of −5.2V by R1 and R3). Also, if the signal SDA_1 has a low logic level (0V), then light is emitted from the light emitting device of optical transceiver 1, and accordingly, the signal SDA_2 goes to a lowlogic level (−5.2V).
However, when a signal is transmitted from the SDA_2 stage to the SDA_1 stage, if the signal SDA_2 has a high logic level (0V), light is not emitted from the light emitting device of optical transceiver 2, and accordingly, the signal SDA_1 goes to a high logic level (a voltage resulting from the division of +3.3V by R2 and R4). Also, if the signal SDA_2 has a low logic level (−5.2V), then light is emitted from the light emitting device of optical transceiver 2, and accordingly, the signal SDA_1 goes to a low logic level (0V).
According to the above-described configuration and operation of the optical transceivers shown in FIG. 2, when VccA is 3.3V and VccB is −5.2V, since one of the optical couplers has a signal (SDA_1) as the LVTTL level by +3.3V and the other of the optical couplers has a signal (SDA_2) by −5.2V, signals according to the I2C communication system can be transmitted and received even at different voltage levels of +3.3V and −5.2V.
In addition, with reference to FIG. 3, the circuit for SCL signal level shift will now be described. Since an SCL signal is a unidirectional signal, it is shown in FIG. 3 that one optical coupler 33 (typically a 6N139 semiconductor) is used for the signal level shift. In transmission of the SCL signal, the first operation voltage VccA (+3.3V) is input to a light emitting device (6N139 terminal 2) of optical coupler 33 via resistor R5 and an output terminal (6N139 terminal 3) of the light emitting device is connected to an SCL_1 stage.
In addition, an SCL_2 stage is connected to a light receiving device of optical coupler 33 and a ground via a resistor R6. The light receiving device of optical coupler 33 is configured to connect/disconnect the SCL_2 stage to/from the second operating voltage VccB (−5.2V), by performing an on/off switching based on the presence or absence of light emitted from the light emitting device.
With regard to the operation of the SCL signal level shift circuit having the configuration as described above, if a signal SCL_1 has a high logic level (+3.3V), light is not emitted from the light emitting device of optical transceiver 33, and accordingly, a signal SDA_2 goes to a high logic level (0V). Also, if the signal SDA_1 has a low logic level (0V), then light is emitted from the light emitting device of optical transceiver 33, and accordingly, the signal SDA_2 goes to low logic (−5.2V).
As described above, although there has is a known circuit that provides a signal level shift using optical transceivers, such optical transceivers commonly are inconvenient in their use because of their relatively large size. For example, in the case of 6N139 chips of ‘Fairchild Semiconductor’ Co., an optical transceiver has a size of 9.91 mm×6.86 mm, which occupies a great deal of space on a substrate, for example, when three or more optical transceivers are mounted on the substrate.